The present invention relates to methods for fabricating materials and structures having mechanical, thermal and electrical properties suitable for use in a wide variety of applications.
Materials fabricated from powder have in the past been fabricated using a sintering process but do not have a sufficient density to provide a product having sufficient mechanical strength or thermal stability.
For example, boron has a wide variety of uses, but it is a difficult material to form in a desired geometry and is also difficult to machine. Arsenic, phosphorus, antimony and boron are all used as dopants in the fabrication of semiconductor devices. These materials are selectively ionized and implanted using an ion implantation system. These systems have an ion source that is used to generate a beam of ionized particles which are directed onto a target such as a semiconductor wafer. These systems are complex and expensive to fabricate, operate and maintain. A particular problem in the use of these ion implanters is the level of impurities generated during use which increases maintenance, increases defect density in the materials produced and reduces production yield in the manufacture of devices.
The housing for the ion source in an implanter is often referred to as an arc chamber. Arc chambers have usually been made of graphite, molybdenum or tungsten. These materials contribute to the contamination of the beam, and consequently, they contaminate the final product.
In one type of arc chamber electrons are emitted by a cathode, usually by thermionic emission, and accelerated to an anode. Some of these electrons have collisions with gas atoms or molecules and ionize them. Secondary electrons from these collisions can be accelerated toward the anode to energies depending on the potential distribution and the starting point of the electron. Ions can be extracted through the anode, perpendicular to it, or through the cathode area depending upon the type of source.
To increase the ionization efficiency of the electrons in electron bombardment ion sources, several modifications have been introduced in existing systems. An additional small magnetic field confines electrons inside the anode and lets them spiral along the magnetic field lines, multiplying on their way to the anode and increasing the ionization efficiency of the ion source. By using a cylindrical anode and a reflector electrode, the electron path is further enlarged. Many mass separator ion sources are this type, such as the Nier, Bernas, Nielsen, Freeman, Cusp and other sources.
The Bernas ion source, for example, has a rectangular or cylindrical arc chamber positioned in an external magnetic field. The source can contain a single-turn helical filament (cathode) at one side of the arc chamber and a reflector at the other end. Electrons from the cathode are confined inside the anode cylinder by the magnetic field and can oscillate between the filament and the reflector resulting in a high ionization efficiency. Ions are extracted perpendicular to the anode axis through a slit of about 2 mm width and about 40 mm length. However, the dimensions can vary, depending on the specific design.
A continuing need exists for improvements in the field of materials fabrication to provide structures having desired mechanical, thermal and electrical properties. In particular, there is a need for improvements in ion implantation systems used for the fabrication of semiconductor devices.